It sounds like you've got a lot of "blowby" past your rings. When was the last time you had a top end overhaul? I've seen all sorts of problems that cause excessive oil to be blown out of the crankcase vent when starting the engine or when it's idling but when it's running, it's usually because it needs a new set of rings in the engine.

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That's common if there's too much oil in the crankcase. You may want to re-check the data you have on how much oil the crankcase is supposed to hold.

It can also happen if there's too much blow-by from cracked rings, a punctured piston, or a badly scored cylinder wall. It can even happen if the valves are badly out of adjustment. But if the engine is running smoothly, it's unlikely to be any of these.

Is it coming from the Breather hose? try taking out the air filter and see if that stops it. If air filter is clogged , it will cause Crank Pressure.Also check oil level if too high it will do this. Sometimes the carb will let gas past and into crankcase filling teh crankcase above 'Full' mark

Oil coming via the engine breather can mean a worn engine, it could be as simple as an over filled sump, or the breather valve itself is not working, the breather valve allows the crankcase pressure out of the engine.

There ae a number of things it could be. What it sounds like to me is that the oil is not being pumped back into the oil tank and is collecting in the cam chest. It should be blowing out of your crankcase breather tube as well if your crankcase breather tube is open. Behind the oil pump, there are three holes. One is behind and above the oil pump. This is where the breather tube from your oil tank and the primary comes into the engine. The hole directly behind the oil pump in the middle is the oil return line from the very back end of the primary where the oil from the primary is recirculated back into the engine. The bottom hole behind the oil pump is the crankcase breather. It should have a hose that runs back around behind the transmission and just dead ends in a bracket pointed towards the final drive chain. This hose should be open to vent the crankcase pressure. Have you checked to see if oil is being pumped back into the oil tank? If not, the scavenge gears of the oil pump may not be pumping the oil out of the engine and back to the tank. If this is the problem, you may not have the drive key properly installed in the scavenge gears of the oil pump. Still, oil should be pouring out of the crankcase breather hole. But, this is what it sounds like it's doing to me. The oil is building up in the cam chest for some reason.

Most of the time the oil problem is caused by the little breater "umbrellas" in center rocker box section. Over time of heating and cooling, the little "umbrella" become brittle and fail to seal properly. Crankcase pressure then blows the oil out of the top of the heads and into the breater.

Most people purchase and install an aftermarket breather kit that reroutes the crankcase pressure. This is fine but it does not stop the oil problem, it just keeps it out of the breather and moves it somewhere that it is not as noticable. The proper fix it to remove the rocker boxes and replace the little umbrella seals.

Also, you say that the oil tank is not overfilled but I've see it thousand times on Sporty's. People will check the oil before a ride and fill the tank to the top line on the stick. Too much oil. The stick has two lines, the lower line is "full cold" and the upper line is "full hot". As long as the oil is between the two lines do not add oil to the tank. I've seen the oil filler plug blow out after the oil gets hot. The oil expands as it heats up. Check the oil when the engine and the oil is hot.

The oil leak normaly would make the 'friction disc' inside the frame slip so as to not drive the axle.
Locate the oil leak
#1 engine crankshaft seal
#2 engine crankcase cover gasket
#3 engine overfull so that the excess oil is coming out of the crankcase breather tube and running down inside the frame